1. Field of the Invention
The present invention relates to a display range control apparatus and an external storage unit for use therewith. More particularly, the present invention relates to a display range control apparatus along with an external storage unit for use in a TV game machine, for example, the display range control apparatus displaying a picture on a screen of a raster scan monitor while masking a range of that screen.
2. Description of the Prior Art
One type of TV game machine is disclosed in Japanese Patent Laid-Open No. 59-68184 (laid open on Jul. 7, 1984 (corresponding to U.S. Pat. No. 4,824,106 issued on Apr. 25, 1989). In such a type of TV game machine, it is conceivable that when texts are displayed throughout a text-based adventure game or when an outdoor scenery as it is viewed from an window of a building or a vehicle is displayed, a window on a screen of a raster scan monitor is formed so as to display a picture within that window alone the rest of the screen is masked.
According to the above prior art method, pictures having the described window effect can be displayed by causing a suitable program to update still picture or background picture pattern data contained in a screen RAM.
One disadvantage of the above prior art method is that the screen RAM must be updated over a plurality of frames because it is impossible to update the whole RAM instantaneously. Such frame-by-frame updating impairs visual effects. Furthermore, the above method has constraints on the window configuration. That is, while a rectangular window only requires the screen RAM to accommodate character codes for black or white characters, a circular window necessitates separately storing characters corresponding to diverse shapes of components making up the circle. To implement this feature would require the character memory to be increased in capacity. Since the TV game machine must be inexpensive so as to be competitive on the market, the machine cannot afford the memory capacity increase and is thus subject to window configuration constraints.
One way to circumvent the above problem is to transfer the character data to a separate RAM which is then updated to vary character shapes. One disadvantage of this solution is that it takes time to update the character RAM. The prolonged update makes the TV game machine impractical.